Those tweets were generated by a tool called Thunderclap, which the White House deployed on its website. Visitors to WhiteHouse.gov were encouraged to sign up with their Twitter and/or Facebook accounts. A suggested -- but customizable -- message was provided. Those signing up were told that the message would be shared at a certain time, and only once.

On Wednesday afternoon, hours before the Senate was set to vote on amendments to its gun bill, the social postings were unleashed:

Thunderclap is a popular tool used by many advocacy groups, but this was the first time that the White House had used it.

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Pivotal Moments In The Federal Gun Control Debate

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1981: The Attempted Assassination Of President Ronald Reagan

on March 30, 1981, President Reagan and three others were shot and wounded in an assassination attempt by John Hinckley, Jr. outside the Washington Hilton Hotel in Washington, D.C. Reagan's press secretary, Jim Brady, was shot in the head.